Process for profile extrusion of a polyester

ABSTRACT

In a process for producing a profile by profile extrusion, a melt of a polyester composition is extruded through a die to form a profile. The processability of the polyester composition is improved by the addition of a branching agent, which provides increased melt strength and increased high shear thinning. The polyester composition has an inherent viscosity of at least 0.65 dl/g. The diacid component of the polyester composition has from 100 to 98.0 mole percent of residues of a primary acid selected from terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid and mixtures thereof. The glycol component of the polyester composition has from 100 to 98.0 mole percent of residues of a primary glycol selected from ethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, neopentyl glycol, and mixtures thereof. The polyester composition has from 0.05 to 2.0 mole percent of residues of the branching agent selected from an acidic branching agent with a tri-functional or greater monomer, an alcoholic branching agent with a tri-functional or greater monomer, and mixtures thereof. The branching agent is present as part of either the diacid component or glycol component depending on whether the functionality is acidic or alcoholic, respectively.

This application claims the benefit of U.S. Provisional ApplicationSerial No. 60/139,285 filed Jun. 15, 1999 titled “Polyester Compositionsfor Profile Extrusion”.

TECHNICAL FIELD OF THE INVENTION

This invention relates to profile extrusion of thermoplastic polymers toform shaped articles commonly referred to as profiles. Moreparticularly, this invention relates to processes of profile extrusionutilizing certain polyester compositions.

BACKGROUND OF THE INVENTION

Thermoplastic polymers are commonly used to manufacture various shapedarticles which may be utilized in applications such as automotive parts,food containers, signs, packaging materials and the like. Profileextrusion is a common, cost-effective method for producing these shapedarticles. Polymers such as polyvinyl chloride (PVC), acrylics andpolycarbonates are typically used in profile extrusion. Each of thesepolymers suffers from one or more disadvantages. PVC is undesirable froman environmental standpoint since PVC produces toxic gases during meltextrusion and is difficult to dispose of after use. Acrylic articles arebrittle and shatter when dropped or struck against another object.Polycarbonate is difficult to work with from a processor's perspectiveand is too expensive for many applications. Polyesters, beingnotoriously difficult to process compared to many other polymers, havenot been utilized as often in profile extrusion. As compared to polymerstypically used in profile extrusion, polyesters have lower meltstrengths and insufficient shear thinning resulting in a greaterpropensity for melt fracture if extruded at high output rates or lowtemperatures. Both melt strength and shear thinning are extremelyimportant from the standpoint of profile extrusion.

Profiles are defined herein by a combination of two factors: shape andprocess of manufacture. The shape of a profile has a particulartwo-dimensional cross-section and an infinite length. The process ofmanufacture is known as profile extrusion. The cross-section lies in thex-y plane and the length lies along the z-axis. The x-y plane usuallycorresponds to the face of the die, whereas the z-axis corresponds tothe extrusion or “take-off” direction. Profiles can take on a widevariety of cross-sections varying in size, shape and complexity. Common“simple” profile shapes include hollow tubes, solid round stock, squarecross-section stock, etc. More complex shapes such as those used forpricing channels, corner guards, and house siding can also be made.

By this use of shape as part of the profile definition, fiber, film andsheet might also be considered as special classes of profiles. Fibershave very small circular cross-sections and are extruded continuously inone direction. Film and sheet have rectangular cross-sections and areextruded continuously. However, in the industry as a whole and asdefined herein by the additional definition factor of process ofmanufacture, film, sheet and fiber are not profiles because of how theyare manufactured. Film or sheet, while infinite in length, aremanufactured by processes that include the use of calendering or chillrolls. Fiber processes involve very high drawdowns, along with spinningcabinets and godet rolls. Profiles, in the industrial vernacular,represent constant cross-section, axially extruded structures, whichhave axial rigidity and are not wound. Profiles are usually cut tolength and bundled, stacked or otherwise bound for transport. This axialrigidity obviously has important implications for what kind of“haul-off” equipment is used to convey the extruded product.Furthermore, the issues of melt strength and melt fracture are notimportant factors in fiber, film and sheet due to the nature of thetake-up/winding equipment and the fact that shape definition is alreadytrivial. Thus, as defined in the industry and herein, “profile” shallnot include fiber, film and sheet.

Profiles are fabricated by melt extrusion processes that begin byextruding a thermoplastic melt through an orifice of a die forming anextrudate capable of maintaining a desired shape. The extrudate istypically drawn into its final dimensions (along the z-axis) whilemaintaining the desired shape (in the x-y plane) and then quenched inair or a water bath to set the shape, thereby producing a profile. Inthe formation of simple profiles, the extrudate preferably maintainsshape without any structural assistance. With extremely complex shapes,support means are often used to assist in shape retention. In eithercase, the type of thermoplastic resins utilized and its melt strengthduring formation is critical. Melt strength is defined as the ability ofa polymer to support its weight in the molten state. For example, whenextruded vertically from a die, a polymer with low melt strength willquickly sag and hit the floor; whereas, a polymer with high meltstrength will maintain its shape for a much longer amount of time.

There are a number of quantitative and qualitative means for measuringmelt strength. One standard test is disclosed in U.S. Pat. No. 4,398,022wherein melt strengths for a polyester used in extrusion blow moldingprocesses were measured at values between −10 and 10 percent. This sametest is utilized herein and involves vertically extruding the polymerfrom a 0.1 inch (0.25 cm) diameter capillary die that is 0.25 inches(0.64 cm) long at a shear rate of 20 s⁻¹ up to a total length of 19inches (49 cm). At this point the strand is cut near the die face andallowed to cool at room temperature. The diameter 6 inches (15 cm) fromthe end of the extrudate is then measured and expressed as a percentagechange relative to the capillary diameter to give the melt strength. Forexample, if the strand diameter at a point 6 inches (15 cm) from thebottom was 0.12 inches (30 cm), then the polymer melt strength at thatgiven melt temperature would be 20 percent (i.e. MS=(0.12−0.1)/0.1*100percent). Similarly, the “die swell” is obtained by measuring thediameter ½ inches (1.3 cm) from the bottom of the extrudate andexpressing it as a percentage change relative to the capillary diameter.

Polyesters due to their poor melt strength may have a negative value forthe melt strength since the 6 inches (15 cm) point diameter could beless than the nominal diameter. For example, linear poly(ethyleneterephthalate) modified with 1,4-cyclohexanedimethanol (PETG) having aninherent viscosity (IV) of 0.76 dl/g has been observed to have a meltstrength of −4 percent at 200° C. and −24 percent at 220° C. This meansthat the diameter of the extrudate measured 6 inches (15 cm) from theend of the strand was 4 percent smaller (200° C. sample) than the dieopening. Typical melt strengths for PVC under standard processingconditions (160 to 200° C. processing temperature) are in the order of20 to 30 percent. To achieve this melt strength with linear PETG wouldrequire an IV of around 0.95 dl/g. Thus, for applications in which meltstrength is critical, polyesters will often not supplant thesecompetitive polymers.

Another common melt strength test involves measuring the time periodthat an extrudate takes to reach a predetermined length below a die fora given flowrate/shear rate. While not standardized, this test providesan easy method for material comparison on a typical processing line andis used in some of the examples cited herein. Other non-standard meltstrength tests such as measuring the degree of drooping in a horizontalprofile extrusion line can also be applied giving a more applicationspecific measure of melt strength.

Profile extrusions are usually run horizontally, and thus melt strengthis important to minimize the amount of “drawdown” and gravity-inducedsagging the polymer experiences upon exiting the die. Drawdown isdefined in profile extrusion as the amount of thickness reductionbetween the die and the take-up system and is expressed as the nominalthickness or width dimension at the die divided by the same dimension inthe final part). For example, a typical polyester drawdown is about two.This means that the width of the final part is ½ that of the width atthe die exit. Similarly, the final thickness is ½ of the thickness atthe die exit. The take-up force of the puller or winder causes drawdownas the melt exits the die. A higher melt strength reduces the amount ofdrawdown, since there is greater resistance to stretching and thinning.For example, the drawdown for higher melt strength PVC is more on theorder of 1.25. Reduced drawdowns make designing the appropriate dies andmaintaining critical final part dimensions much easier.

The inadequate melt strength of polyesters further results in severeprocessing problems when polyesters are processed at typical profileextrusion temperatures of 390-550° F. (200-290° C.) and line speeds.Processing line speeds vary considerably depending on the shape of theprofile. Typical speeds for simple shapes like a corner guard may befrom 50 to 70 feet (15 to 20 meters) per minute. More complicated shapesmay have process line speeds as low as one foot (0.3 meters) per minute,whereas extremely simple shapes with certain types of processingtechnology may run at speeds as high as 100 feet (30 meters) per minute.At the higher speeds, which obviously would be preferred by profilemanufacturers, inadequate melt strength produces an extrudate that doesnot maintain its shape prior to quenching, and thus deformation occurs.To increase the melt strength of the polyester, processing temperaturesare often lowered. This, however, increases the likelihood of anundesirable phenomenon known as melt fracture, which can only beeliminated by lowering the extrusion speed. By decreasing speed, theeconomic attractiveness of using polyesters is also decreased. Thus, theprofile extrusion processes are often operated at maximum speedsassociated with the highest temperatures and minimal melt strengths formaintaining particular profile shapes. Any increase in speed or loweringof temperature may cause an increase in high shear viscosity in the die,which then may cause an undesirable melt fracture.

From a Theological standpoint, melt strength depends primarily on theviscosity and, to some degree, on the elasticity or relaxation time ofthe melt. A higher viscosity increases the resistance todrawdown/sagging. In contrast, melt elasticity causes an increase in dieswell, which serves to offset some of the effects of drawdown. Eventhough the same amount of width/thickness reduction is occurring afterthe die, the highly elastic material starts with a much higher initialwidth/thickness due to the greater die swell. Thus, the final partdimensions remain closer to the die dimension and the effective drawdownseems lower (thereby making it easier to design the tooling needed).

A viscosity curve for a given polymer has two regions of interest, asshown in FIG. 1. At very low shear rates the viscosity is highest andthis is referred to as the “zero shear viscosity”, η₀. The zero shearviscosity (along with the elasticity) defines the melt strength sincethe polymer is experiencing essentially a zero shear rate after exitingfrom the die. Thus, the higher the zero shear viscosity, the higher themelt strength. In the high shear rate region, the polymer is “processed”with shear rates in the die/extruder ranging anywhere from about 10 s⁻¹to 1000 s⁻¹. As low of a viscosity as possible in this range is desiredin order to minimize pumping pressure and melt fracture. Fortunately,most polymers exhibit at least some degree of viscosity reduction or“shear thinning” at higher shear rates, which aids in their“processability”. Without the shear thinning, an extruder running a highmelt viscosity polymer would require extremely high motor loads and/orvery high melt temperatures, both of which can lead to polymerdegradation and excessive energy consumption. In general, condensationpolymers like polycarbonates and polyesters have a very low degree ofshear thinning relative to addition type polymers like PVC andpolyolefins. This is because the condensation polymers typically havenarrower molecular weight distributions in addition to lacking the highmolecular weight “tail” common in many addition polymers. This narrowmolecular weight distribution makes polyesters more “Newtonian-like”(i.e. having a flat viscosity which does not depend much on shear rate)and characteristically harder to process.

Having a low viscosity at high shear rates (i.e. in the die) also servesto minimize the formation of melt fracture or “sharkskin” on the surfaceof the extruded part or article. Melt fracture is a flow instabilityphenomenon occurring during extrusion of thermoplastic polymers at thefabrication surface/polymer melt boundary. The occurrence of meltfracture produces severe surface irregularities in the extrudate as itemerges from the orifice. The naked eye detects this surface roughnessin the melt-fractured sample as a frosty appearance or matte finish asopposed to an extrudate without melt fracture that appears clear.

Melt fracture occurs whenever the wall shear stress in the die exceeds acertain value (typically 0.1 to 0.2 MPa). In turn, shear stress iscontrolled by the volume throughput or line speed (which dictates theshear rate) and the viscosity of the polymer melt. By reducing eitherthe line speed or the viscosity at high shear rates, the wall shearstress is reduced and the chance for melt fracture is lowered. Thus, byincreasing the degree of shear thinning, the viscosity is reduced athigh shear rates, which then allows higher line speeds before meltfracture occurs.

Coupling all of these desired properties together, the ideal polymer forprofile extrusion clearly will have a high zero shear viscosity inconjunction with a high degree of shear thinning. This maximizes meltstrength while at the same time minimizes melt fracture and diepressures. A representative “ideal” curve is illustrated in FIG. 1.

With respect to polyesters, either melt strength may be increased ormelt fracture reduced without significantly affecting a change in theother. For example, by increasing the molecular weight or inherentviscosity of the polyester or by lowering the melt temperature, the zeroshear viscosity will increase significantly along with the meltstrength, but the degree of shear thinning will only change slightly.Thus, the melt strength will increase, but melt fracture will becomeeven more of a problem since the high shear rate viscosity alsoincreased significantly. This is in fact a problem for profileextrusions with polyesters and copolyesters. For example, in order toachieve an acceptable level of melt strength (i.e. greater thanapproximately 0 percent melt strength), PETG must be extruded at a melttemperature of 200° C. or lower. This is an extremely low processingtemperature for a polymer normally designed to be run at 220° C. orhigher. When run at this low temperature, PETG exhibits severe meltfracture even at low line speeds because the high shear rate viscosityis so high. This is due to the lack of significant shear thinning inPETG. Thus, with polyesters the process must be operated at either (1)hotter melt temperatures resulting in no melt fracture but too low meltstrength or (2) low temperatures resulting in adequate melt strength butproblems with melt fracture. Either scenario represents an economicallyunacceptable alternative for profile extrusion, so an improvedprocessing polyester is needed that has both good melt strength and ahigh degree of shear thinning (i.e. melt fracture resistance).

Nevertheless, for some applications (e.g. flat film casting, extrusionblow molding, and foam extrusion) where the effective shearrates/stresses are already fairly low due to bigger die gaps and/orreduced extrusion rates, melt fracture may not be an issue. Thus,increasing just the melt strength alone may be acceptable. However, forprofile extrusion, which typically has very high shear rates, bothincreased melt strength and resistance to melt fracture are importantfactors and should be improved simultaneously.

Chain branching is one of the most commonly used methods for improvingthe melt strength of a polymer, particularly polyesters. A tri-, tetra-,or higher functionality monomer is added to the polyester either duringmanufacture or in the processing step to create branches in the polymer.Typical branching agents for polyesters include trimellitic anhydride(TMA), pyromellitic dianhydride (PMDA), glycerol, sorbitol, hexanetriol-1, 2, 6, pentaerythritol, trimethylolethane, and trimesic acid.Common applications for high melt strength polyesters include extrusionblow molding and foams.

U.S. Pat. No. 4,983,711 to Sublett describes high melt strengthpolyesters for extrusion blow molding applications. The polyester ispoly(ethylene terephthalate) modified with 1,4-cyclohexanedimethanol(PETG) having from 0.05 to 1 mole percent of a tri-functional brancher,preferably trimellitic acid or anhydride. The 1,4-cyclohexanedimethanol(CHDM) levels are from 25 to 75 mole percent.

U.S. Pat. Nos. 5,523,382 and 5,442,036 to Beavers describe branchedcopolyesters suitable for extrusion blow molding. The branching agent ispreferably trimellitic acid or anhydride. The copolyester contains anethylene glycol component modified with 0.5 to 10 mole percent of CHDMand 3 to 10 mole percent of diethylene glycol. The acid component isterephthalic acid with up to 40 percent of isophthalic acid andnaphthalenedicarboxylic acid.

U.S. Pat. No. 5,376,735 to Sublett describes high melt strengthpoly(ethylene terephthalate) (PET) for use in extrusion blow moldingapplications. The PET was blended with up to 3 mole percent isophthalicacid (IPA). A number of branching agents are mentioned including TMA.

Fukuda et. al. in U.S. Pat. No. 5,382,652 (reissued as RE35939)discloses a polyester resin branched with a range of tri- andtetra-functional materials including TMA and others. The resincomposition is from 90 to 100 percent ethylene glycol and from 0 to 10percent of one or more of the following: diethylene glycol, CHDM,propylene glycol, and butanediol. The application described is forimproved processability around film extrusion, molding, andheat-sealability.

In U.S. Pat. No. 5,235,027, Thiele discloses a modified PET forextrusion blow molding. The PET contains from 0.5 to 5 wt. percent ofisophthalic acid, 0.7 to 2.0 wt. percent of diethylene glycol, 300-2500ppm tri- or tetra-hydroxyalkane, 80-150 ppm antimony, phosphorous of atleast 25 percent by weight of the amount of antimony, red and blue toner(not exceeding 5 ppm), and various branching agents with pentaerythritolpreferred. The resultant polyester has an IV between 0.8 and 1.5 dl/g.

Hauenstein in U.S. Pat. No. 4,182,841 describes a modified PETcontaining 12 mole percent neopentyl glycol terephthalate and 0.0062mole percent of a polyfunctional modifying branching agent, includingTMA.

Edelman et al, in U.S. Pat. Nos. 4,234,708, 4,219,527 and 4,161,579disclose various high melt strength polyesters for extrusion blowmolding. A variety of chain branching agents are utilized in amounts offrom 0.025 to 1.5 mole percent with 0.25 to 10 equivalents of a chainterminator, which controls reaction conditions and prevents gelling. Theimportance of high zero shear viscosity coupled with shear sensitivityis also described.

Thus, there exists a need in art to have a polyester composition for usein profile extrusion which has both high melt strength to preventsagging and excessive drawdown and a high degree of shear thinning toresist melt fracture at high processing speeds. Accordingly, it is tothe provision of such that the present invention is primarily directed.

SUMMARY OF THE INVENTION

A process for producing a profile by profile extrusion comprisesproviding a melt of a polyester composition, which has a branching agenttherein and an inherent viscosity of at least 0.65 dl/g, at a melttemperature and extruding the melt through a die to form a profile. Thediacid component of the polyester composition comprises 100 to 98.0 molepercent of residues of a primary acid selected from the group consistingof terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid andmixtures thereof. The glycol component of the polyester compositioncomprises 100 to 98.0 mole percent of residues of a primary glycolselected from the group consisting of ethylene glycol,1,4-cyclohexanedimethanol, diethylene glycol, neopentyl glycol, andmixtures thereof. The polyester composition comprises from 0.05 to 2.0mole percent of residues of the branching agent selected from an acidicbranching agent with a tri-functional or greater monomer, an alcoholicbranching agent with a tri-functional or greater monomer, and mixturesthereof. The branching agent is present as part of either the diacidcomponent or glycol component depending on whether the functionality isacidic or alcoholic, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical viscosity versus shear rate curve for a standardpolyester and an “ideal” resin having improved processability.

FIG. 2 is a photograph of actual profiles in their sagged state atconstant throughput for PETG resins having different inherentviscosities and the same branching level.

FIG. 3 is a viscosity curve for branched PETG at 230° C. and 240° C. andneat PETG at 230° C.

FIG. 4. is a viscosity curve at 270° C. for copolyesters modified witheither 24 mole percent or 35 mole percent isophthalic acid andcontaining no or 0.15 percent branching agent.

DETAILED DESCRIPTION OF THE INVENTION

Polyester compositions as alternative materials for use in makingprofiles have heretofore been difficult to incorporate into profileextrusion processes. As discussed above, to obtain the requisite meltstrength, polyesters are typically processed at lower temperatures andthus lower speeds to prevent melt fracture caused by the lack of shearthinning. The ideal polyester would thus have a high melt strength and ahigh degree of shear thinning so that the polyester could be run at thehigh speeds associated with profile extrusion without sagging and meltfracture.

The present invention addresses the problems of the prior art byproviding a process for producing a profile by profile extrusion whereina melt of a polyester composition, which has been modified by theaddition of a tri-functional or greater branching agent, is extrudedthrough a die to form a profile. Unexpectedly, this polyester providesboth characteristics of high melt strength and a high degree of shearthinning making it suitable for use in profile extrusion processes. Thebranching agent imparts higher melt strength and a higher degree ofshear thinning (i.e. resistance to melt fracture) to the polyesterresulting in easier startup in the profile line, higher line speeds andbetter control of the final profile dimensions.

The polyester compositions of the present invention have an inherentviscosity (IV) of at least 0.65 dl/g, preferably at least 0.7 dl/g,providing the high melt strength necessary for profile extrusion. Thepolyester composition is based on 100 mole percent of a diacid componentand 100 mole percent of a glycol component. A branching agent is presenteither as a portion of the mole percentage of the diacid component orthe glycol component depending, respectively, on whether the branchingagent has acid functionality or alcohol functionality as describedbelow.

The polyester composition comprises a diacid component of from 100 to98.0 mole percent, preferably 99.9 to 99.0 mole percent, of residues ofa primary acid selected from terephthalic acid (TPA), isophthalic acid(IPA), naphthalenedicarboxylic acid (NDA) or mixtures thereof.Preferably, TPA is the predominant primary acid (at least 60 molepercent) with up to 40 mole percent of IPA therein. A more preferredembodiment has TPA as the sole primary acid.

The primary acid of the diacid component may optionally be modified byreplacing one or more of TPA, IPA and NDA with up to 40 mole percent ofone or more different diacids, such as saturated aliphatic dicarboxylicacids having 4 to 12 carbon atoms and cycloaliphatic dicarboxylic acidshaving 8 to 12 carbon atoms. Specific examples of diacids are: phthalicacid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid,diphenyl-4,4′-dicarboxylic acid, succinic acid, glutaric acid, adipicacid, azelaic acid, sebacic acid, and the like. It should be understoodthat use of the corresponding acid anhydrides, esters, and acidchlorides of these acids is included in the term “diacid”.

The polyester composition comprises a glycol component of from 100 to98.0 mole percent, preferably 99.9 to 99.0 mole percent, of residues ofa primary glycol selected from ethylene glycol (EG),1,4-cyclohexanedimethanol (CHDM), diethylene glycol (DEG), neopentylglycol (NPG), or mixtures thereof. A preferred level of EG and CHDM isfrom 0 to 100 mole percent, whereas a preferred level of NPG is from 0to 40 mole percent. A small amount of DEG is usually also present as aside reaction in the manufacturing process, although it can be increasedto higher levels if desired. A typical range for DEG is from 0 to 10mole percent, with a preferable range from 0 to 3 mole percent. The mostpreferred embodiments have 57 to 90 mole percent EG with 10 to 40 molepercent of either CHDM or NPG and/or 0 to 3 mole percent of DEG.

In addition, the primary glycol of the glycol component can be modifiedwith up to 40 mole percent of one or more different diols. Suchadditional diols include cycloaliphatic diols having 6 to 15 carbonatoms and aliphatic diols having 3 to 8 carbon atoms. Examples of suchdiols are: triethylene glycol, propane-1,3-diol, butane-1,4-diol,pentane-1,5-diol, hexane-1,6-diol, 3-methylpentanediol-(2,4),2-methylpentanediol-(1,4), 2,2,4-trimethylpentane-diol-(1,3),2-ethylhexanediol-(1,3), 2,2-diethylpropane-diol-(1,3),hexanediol-(1,3), 1,4-di-(hydroxyethoxy)-benzene,2,2-bis-(4-hydroxycyclohexyl)-propane,2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane,2,2-bis-(3-hydroxyethoxyphenyl)-propane,2,2-bis-(4-hydroxypropoxyphenyl)-propane, and the like.

The polyester composition further comprises from 0.05 to 2.0 molepercent, preferably 0.1 to 1.0 mole percent, of residues of a branchingagent selected from an acidic branching agent comprising atri-functional or greater monomer, an alcoholic branching agentcomprising a tri-functional or greater monomer, or mixtures thereof. Theacidic branching agent refers to branching agents containing acidmoieties that react within the polyester chain. The alcoholic branchingagent refers to branching agents containing alcohol moieties that reactwithin the polyester chain. The branching agents comprising atri-functional or greater monomer have at least three functional groupsthat can react to create branches in the polyester chain. Preferredbranching agents include trimellitic anhydride (TMA), pyromelliticdianhydride (PMDA), glycerol, sorbitol, hexane triol-1,2,6,pentaerythritol, trimethylolethane, and trimesic acid. When the acidicbranching agent is used in the present invention, the mole percentage ispart of the acid component. When the alcoholic branching agent is used,the mole percentage is part of the glycol component.

In a preferred embodiment of this invention, TMA is present from 0.1 to2 mole percent, based on 100 mole percent of the diacid component.Because of the difficulties in processing the higher levels of branchingdue to high reactor viscosities, in addition to the problem of gelformation, the most preferred level of TMA is from 0.1 to 0.35 molepercent. Below 0.1 percent TMA, the degree of processability improvementdoes not warrant the added cost of introducing the branching agent;whereas, above about 0.3 mole percent the reaction rate in a productionfacility becomes more difficult to control unless chain terminators areadded. Chain terminators suitable for use in the present invention aremonfunctional with an acid or alcohol functionality. Examples includestearic acid and benzoic acid. If the branching agent is tri-functional,the chain terminator and branching agent are preferably present at aratio of 3:1 and more preferably at a ratio of 1:1. If the branchingagent is tetra-functional, the chain terminator and branching agent arepreferably present at a ratio of 4:1 and more preferably at a ratio of2:1.

The melt strength of the polyester composition used in the profileextrusion process is preferably greater than about 0 percent at aparticular melt temperature utilized in the process. Typical processingtemperatures of the polyesters of the present invention are about 220 to230° C. A “standard” profile should be processed without too muchdifficulty at about 0 percent melt strength. More complicated shapesmight require a melt strength greater than about 20 percent, whereassimpler parts could be run adequately at melt strengths below 20percent. As mentioned previously, melt strength is a strong function ofthe melt temperature, IV and, as described in this invention, the degreeof branching agent. The melt strength as used herein is determined byusing the test described in U.S. Pat. No. 4,398,022.

The polyester compositions utilized in the process of the presentinvention are further characterized by their high degree of shearthinning, i.e. having a low high shear rate viscosity, resulting in aprofile that does not suffer from the phenomenon known as melt fracture.The “power law index, n” can be used to quantify the shear thinning.With reference to FIGS. 1 and 3, the slope of the viscosity versus shearrate curve in the high shear rate region, on a log-log plot, is equal to“n−1”. If n is equal to 1 as in a purely Newtonian case, then the slopeis equal to 0 and there is no shear thinning. As n gets smaller andsmaller, the slope, or degree of shear thinning gets greater and greaterwith n=0 being the ultimate limit. To determine the power law indices,the slope is measured for a given curve over the region from about ω=100to ω=400 wherein ω is the effective shear rate or rheometer testfrequency, which is approximately equivalent to the shear rate. In FIG.3, neat PETG (IV of 0.76 dl/g) at 230° C. has a value of n equal to0.64. Typical values for linear polyesters range from 0.6 to 0.7depending on IV and error in the measurement. PETG with 0.15 molepercent TMA (IV of 0.76 dl/g) at 230° C. and 240° C., embodiments of thepresent invention, have values of n=0.56 and n=0.57, respectively. Thus,in a preferred embodiment, the polyesters used in the process of thepresent invention have a power law index less than 0.6, more preferablyless than 0.575, and even more preferably less than 0.5. As acomparison, other typical polymers used in profile extrusion have evenlower values of n. The exact value depends on the type of polymer andhow the polymer is made. For example, low density polyethylene, linearlow density polyethylene, and metallocene are all polyethylenes thatvary in n due to differences in branching and molecular weightdistribution. Similarly, all of the so-called “easy to process” resinssuch as PVC and polystyrene have low values as well, typically less than0.5.

The occurrence of melt fracture in a profile, as previously discussed,is typically a visual determination. Cloudy appearance is an indicationof melt fracture, and a smooth clear appearance is an indication of nomelt fracture. To quantify this lack of melt fracture in the resultingprofile of the present invention, a surface roughness measurement isutilized. Surface roughness measurements are made using a Dektak3profilometer over a scan length of 0.39 inches (10 mm) along the axialdirection of the profile. For comparison with the visual determination,the RMS surface roughness measurement of a profile made using linearPETG having visible melt fracture is approximately 17000 Angstroms. Thesurface roughness took the form of periodically spaced bumps on thesurface having a “wavelength” of approximately 1000 microns. Thisperiodicity is typical of melt fracture due to the stick-slip behavior.The surface roughness for a profile made using the polyester of thepresent invention having a smooth appearance, i.e. no melt fracture, isapproximately 400 to 500 Angstroms. The actual surface roughness of a“smooth” profile will vary depending on the roughness of the dietooling, cooling rate, etc. even if no melt fracture is present. Thus,profiles produced according to the present invention preferably have asurface roughness measurement of 5000 Angstroms (RMS) or less. Morepreferably, the surface roughness is 1000 Angstroms or less and evenmore preferably is 500 Angstroms or less.

The polyesters can be made via direct esterification of the acid andglycol, or through ester exchange of the glycol with the correspondingdimethyl ester (e.g. dimethyl terephthalate (DMT) in place of TPA ordimethyl isophthalate (DMA) in place of IPA). When using TMA as thebranching agent and EG as one of the glycol components, the TMA ispreferably pre-reacted with ethylene glycol before adding to thereactor. This reduces the amount of residual TMA left in the finalproduct and minimizes the amount of TMA related extractables.

The branching agent may be added in the form of a concentrate ormasterbatch that could be dry blended with the neat polyester justbefore entering the extruder. Nevertheless, the level of branching agentwould have to be much higher (>0.5 mole percent) with the exact leveldepending on the masterbatch letdown ratio. Because of the higher levelof branching agent in the concentrate, reaction conditions would have tobe carefully controlled. However, this approach may not achieve the samebroad MW distribution (thus reducing its effectiveness) compared to acondensation reactor grade product.

Inherent Viscosity (IV) as used herein is measured at 25° C. in asolvent mixture consisting of 60 percent by weight phenol and 40 percentby weight tetrachloroethane.

This invention can be further illustrated by the following examples ofpreferred embodiments thereof, although it will be understood that theseexamples are included merely for purposes of illustration and are notintended to limit the scope of the invention unless otherwisespecifically indicated.

EXAMPLES

There are a number of quantitative and qualitative means for measuringmelt strength. The standard test used for this invention is thatdisclosed in U.S. Pat. No. 4,398,022 and set forth herein above. Anothercommon melt strength test involves measuring the time for the extrudateto. reach a predetermined length at a given flowrate/shear rate. Whilenot standardized, this test provides an easy method for materialcomparison on typical processing lines and is used in some of theexamples. Other non-standard melt strength tests such as measuring thedegree of drooping in a horizontal profile extrusion line can also beapplied giving a more application specific measure of melt strength.

Melt fracture is more difficult to measure quantitatively and nostandardized test has been developed. This is because a moderatelymelt-fractured part typically has a hazy, rough surface, which isimmediately noticeable to the naked eye. Thus, visible melt fracturemakes a profile extrusion unacceptable, particularly for polyesters,where it is usually desired that the profile be clear. Nevertheless,optical transmission and surface roughness measures, as alreadymentioned, can be applied effectively although visual observation isstill the most common method.

Example 1

Film Extrusion Comparison of PETG and 0.3 Mole Percent TMA Modified PETG

An unbranched PETG with 31 mole percent modification of CHDM and IV of0.76 dl/g (Control 1) and a branched PETG with 31 mole percentmodification CHDM, 0.30 mole percent TMA and an IV of 0.715 (Example 1)were extruded on a 1″ (2.54 cm) Killion extruder (24:1 UD, Maddockmixing section screw) using a 6″ (15.2 cm) film die, a 12 (0.3 mm) milgap and a 60 RPM screw speed. Extrusion temperatures were the same forboth polymers (feed zone temperature=21° C., all other zones at 230°C.). At a time denoted as “time zero”, the die was scraped, and thepolymer then allowed to flow until such a point that it contacted thefloor (approximately a 4 foot (1.2 m) height). This time was denoted asthe “sag time”.

For the Control 1, the sag time was 20+/−2 seconds, whereas for Example1, the sag time was 31+/−1 seconds. Thus the melt strength of Example 1was about 50 percent higher than the Control 1 by this measure. The diepressures for the Control 1 and Example 1 were 1480 psi (10 MPa) and1400 psi (9.6 MPa), respectively. Similarly the screw loads were 14 and8.9 amps, respectively. The die pressures and screw loads are indicativeof the shear thinning and ease of extrusion. Clearly, the TMA increasesthe melt strength while reducing the energy and pressure required topump the material (a lower pressure indicates a lower probability formelt fracture since it relates to the wall shear stress).

Example 2

Profile Extrusion Trials with PETG Samples

Different combinations of TMA vs. polyester resin IV samples wereprocessed on a profile extrusion line and tested for melt strength.Since there was no known method for quantifying melt strength online, amethod was devised (referred to herein as the “Sag Test”) using a dieand calibration tooling setup designed to make flat, thin (at die exit)profile shapes. The dimensions of the extrudate at the die exit was 3inches (7.6 cm) by 0.080 inches (0.2 cm). Running at 30 ft/min (9.1m/min) line speed, PETG with 31 mole percent modification of CHDM(Control 2) was extruded at conditions (temperature and screw speed,etc.) which were on the verge of melt fracture so as to establish apoint (aesthetically) at which to set conditions for the test. Afterallowing the profile line to run for an hour in order to reachequilibrium conditions, the calibration tooling was gradually distancedfrom the die. This was done until 1) the face of the calibration toolingwas exactly 20 inches (50.8 cm) from the face of the die or 2) theextrudate “sagged” to the point of not being able to run thus causing asystem shutdown.

During this “Sag Test”, the Control 2 extrudate often sagged to thefloor causing system shutdown and thus failed the Sag Test. Thepolyester compositions of the present invention did not fall to thefloor, but rather continued to run allowing for measurement of theamount of sag, and thus passed the Sag Test. A video camera was fixed 3feet (92 cm) from the sagging extrudate for each sample. Picturessimilar to that shown in FIG. 2 were taken. The point where theextrudate exits the die is 43.5 inches (110.5 cm) from the floor(referred to herein as the “Zero Sag” point). This is also the sameheight where the extrudate enters the water tank. Once each sample(while sagging) reached a state where maximum sag had been reached, thenumber of inches from the floor to the bottom of the sagging extrudatewas recorded. This measurement was then subtracted from the Zero Sagpoint to give a Sag Value for each sample. Lower Sag Values indicatehigher melt strength materials.

FIG. 2 is an image showing superimposition of actual extrudates in theirsagging state. The samples in FIG. 2 were made using the composition ofControl 2 and containing 0.15 mole percent TMA as the branching agent.Sample A (IV of 0.80 dl/g) had a Sag Value of 2.47 inches (6.27cm) amelt temperature of 229° C. Sample B (IV of 0.76 dl/g) had a Sag Valueof 3.34 inches (8.48cm) at a melt temperature of 226° C. Sample C (IV of0.74 dl/g) had a Sag Value of 5.91 inches (15.01cm) at a melttemperature of 224° C.

Table 1 below contains Sag Value data for other samples of polyestercompositions with varying amounts of TMA and values of IV.

TABLE 1 Sample: % TMA 0% 0.1% 0.1% 0.21% 0.135% 0.1% 0.215% and IV 0.760.76 0.78 0.74 0.78 0.80 0.785 Sag Value (in) Failed 8.13 7.38 5.44 4.132.38 1.88 Sag Value (cm) Failed 20.65 18.75 13.82 10.49 6.05 4.78 MeltTemp (° C.) 224 227 227 224 225 226 229

Example 3

Rheological Data for Branched and Unbranched PETG Having a 0.76 IV

The melt strength test utilized in Example 2 was developed specificallyfor profile extrusion. The degree of drooping in a horizontal profileextrusion line between the die and the water bath for a given distanceof separation is measured.

Samples of PETG having an IV of 0.76 dl/g were prepared both with andwithout TMA branching agent. The PETG has a 31 mole percent modificationof CHDM. The viscosity curves for the branched PETG (0.15 mole percentTMA) at 230° C. and 240° C. and for the unbranched PETG at 230° C. areshown in FIG. 3. If the ratio of the viscosity at 1 rad/s over theviscosity at 400 rad/s is used as a “processability index”, theunbranched resin has a processability index of 2.9 whereas the branchedresin has an index of 7 measured at the same temperature. Thus, thebranched resin will be much easier to process and have better meltstrength than the unbranched resin. Additionally, by increasing thetemperature of the branched resin by 10° C., both melt strength andshear thinning properties of the resin were enhanced.

Example 4

Melt Strength Data for Branched and Unbranched PETG Having a 0.76 IV

Using the same samples as in Example 3, melt strength measurements wereperformed at a temperature of 220° C. using a capillary rheometer andthe standard protocol outlined earlier. The unbranched PETG had a meltstrength of −26 percent at 220° C., whereas the 0.15 mole percent TMAbranched PETG had a melt strength of 4 percent. Clearly the branchedPETG is much better suited for profile extrusions at this temperaturebecause its melt strength is greater than 0 percent. The linear PETGwould have to be run much colder to achieve the same melt strength,thereby increasing the likelihood of melt fracture.

Example 5

Branched and Unbranched Copolyesters Containing Neopentyl Glycol

Samples of a copolyester of containing a glycol component of 37 molepercent of neopentyl glycol (NPG) and 63 mole percent of ethylene glycoland a diacid component of terephthalic acid were produced both with andwithout branching agent to an IV of 0.70 dl/g. Branching levels were 0,0.15 or 0.30 mole percent of TMA. The measurements for viscosity were at230° C. The zero shear viscosity was measured at a shear rate of 1 rad/sand the high shear viscosity was measured at a shear rate of 400 rad/s.As clearly indicated by the data in Table 2 below, the use of branchingagents significantly improves the processability of NPG basedcopolyesters.

TABLE 2 Sample % zero shear high shear Branching viscosity, viscosity,process- power Agent poise poise ability index law index   0% TMA 180007100 2.5 0.64 0.15% TMA 54000 8100 6.7 0.53 0.30% TMA 78000 7800 10 0.47

Example 6

Branched and Unbranched Copolyesters Containing Isophthalic Acid

Samples of a copolyester containing 100 mole percent CHDM as the glycolcomponent and a diacid component of either 24 or 35 mole percent ofisophthalic acid (IPA) and correspondingly 76 or 65 mole percent ofterephthalic acid, respectively, were produced both with and withoutbranching agent. The viscosity curves of the four samples are depictedin FIG. 4. As observed in FIG. 4, the degree of shear thinning is muchgreater in the branched samples making them more processable. The powerlaw index for the four samples are set forth in Table 3. While theseexamples of branched IPA modified resins have a power law index ofgreater than 0.6, which is a preferred embodiment of the presentinvention, the data indicates that branch does improve processability ofthe resins making them more suitable for use in profiles. The dataindicates that a higher level of branching agent would be required tolower the power law index.

TABLE 3 Sample Description n 24% IPA, 0% TMA 0.83 24% IPA, 0.15% TMA0.74 35% IPA, 0% TMA 0.83 35% IPA, 0.15% TMA 0.77

Thus, the present invention provides polyester compositions that aresuitable for use in making profiles by profile extrusion. Thesepolyesters have higher melt strengths and higher shear thinning thanlinear polyesters making them extremely suitable for profile extrusionprocesses.

We claim:
 1. A process for producing a profile by profile extrusioncomprising the steps of: (a) providing a polyester composition at a melttemperature, wherein the polyester composition has an inherent viscosityof at least 0.65 dl/g and comprises (1) a diacid component comprising100 to 98.0 mole percent of residues of a primary acid selected from thegroup consisting of terephthalic acid, napthalenedicarboxylic acid,isophthalic acid and mixtures thereof; (2) a glycol component comprising100 to 98.0 mole percent of residues of a primary glycol comprising 57to 90 mole percent ethylene glycol, 10 to 40 mole percent1,4-cyclohexanedimethanol or neopentyl glycol, and 0 to 3 mole percentdiethylene glycol; and (3) 0.05 to 2.0 mole percent of residues of abranching agent selected from the group consisting of an acidicbranching agent comprising tri-functional or greater monomer, analcoholic branching agent comprising tri-functional or greater monomer,and mixtures thereof; wherein the polyester composition is based on 100mole percent of the diacid component and 100 mole percent of the glycolcomponent; and the acidic branching agent comprises a portion of thediacid component and the alcoholic branching agent comprises a portionof the glycol component; (b) extruding the melt through a die to form aprofile.
 2. The process of claim 1 wherein the polyester composition hasa melt strength of greater than 0 percent at the melt temperature. 3.The process of claim 2 wherein the melt strength at the melt temperatureis greater than 20 percent.
 4. The process of claim 1 wherein a powerlaw index of the polyester composition is less than 0.6.
 5. The processof claim 4 wherein the power law index is less than 0.575.
 6. Theprocess of claim 5 wherein the power law index is less than 0.5.
 7. Theprocess of claim 1 wherein the profile has a surface roughness of 5000Angstroms or less.
 8. The process of claim 7 wherein the surfaceroughness is 1000 Angstroms or less.
 9. The process of claim 8 whereinthe surface roughness is 500 Angstroms or less.
 10. The process of claim1 wherein the branching agent is present from 0.1 to 1.0 mole percent.11. The process of claim 1 wherein the primary acid comprises at least60 mole percent terephthalic acid and up to 40 mole percent isophthalicacid.
 12. The process of claim 1 wherein the primary acid is modified bysubstituting one or more of the primary acids with up to 40 mole percentof a dicarboxylic acid selected from the group consisting of saturatedaliphatic dicarboxylic acids having 4 to 12 carbon atoms, cycloaliphaticdicarboxylic acids having 8 to 12 carbon atoms, and mixtures thereof.13. The process of claim 12 wherein the dicarboxylic acid is selectedfrom the group consisting of phthalic acid, cyclohexanedicarboxylicacid, cyclohexanediacetic acid, diphenyl-4,4′-dicarboxylic acid,succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid,and mixtures thereof.
 14. The process of claim 1 wherein the branchingagent is selected from the group consisting of trimellitic anhydride,pyromellitic dianhydride (PMDA), glycerol, sorbitol, hexane triol-1,2,6,pentaerythritol, trimethylolethane, trimesic acid, or mixtures thereof.15. The process of claim 1 wherein the acidic branching agent istrimellitic anhydride and is present in the amount of 0.1 to 0.35 molepercent.
 16. The process of claim 1 wherein the primary glycol ismodified with up to 40 mole percent of diols selected from the groupconsisting of cycloaliphatic diols having 6 to 5 carbon atoms, aliphaticdiols having 3 to 8 carbon atoms, and mixtures thereof.
 17. The processof claim 1 wherein the branching agent is tri-functional, the polyestercomposition further comprises a chain terminator of a monofunctionalacid or alcohol, and the chain terminator and branching agent arepresent at a ratio of 3:1.
 18. The process of claim 17 wherein the chainterminator is selected from the group consisting of stearic acid andbenzoic acid.
 19. The process of claim 1 wherein the branching agent istetra-functional, the polyester composition further comprises a chainterminator of a monofunctional acid or alcohol, and the chain terminatorand branching agent are present at a ratio of 4:1.
 20. In a process forproducing a profile by profile extrusion of a thermoplastic resinwherein a melt of the thermoplastic resin at a melt temperature isextruded through a die to from the profile, the improvement comprisingusing as the thermoplastic resin a polyester composition having aninherent viscosity of at least 0.65 dl/g and comprising: (a) a diacidcomponent comprising 100 to 98.0 mole percent of residues of a primaryacid selected from the group consisting of terephthalic acid,napthalenedicarboxylic acid, isophthalic acid and mixtures thereof; (b)a glycol component comprising 100 to 98.0 mole percent of residues of aprimary glycol comprising 57 to 90 mole percent ethylene glycol, 10 to40 mole percent 1,4-cyclohexanedimethanol or neopentyl glycol, and 0 to3 mole percent diethylene glycol; and (c) 0.05 to 2.0 mole percent ofresidues of a branching agent selected from the group consisting of anacidic branching agent comprising tri-functional or greater monomer, analcoholic branching agent comprising tri-functional or greater monomer,and mixtures thereof; wherein the polyester composition is based on 100mole percent of the diacid component and 100 mole percent of the glycolcomponent and the acidic branching agent comprises a portion of thediacid component and the alcoholic branching agent comprises a portionof the glycol component.
 21. The process of claim 20 wherein thepolyester composition has a melt strength of greater than 0 percent atthe melt temperature.
 22. The process of claim 20 wherein a power lawindex of the polyester composition is less than 0.6.
 23. The processclaim 20 wherein the profile has a surface roughness of 5000 Angstromsor less.